Integrated services digital networks (ISDNs) are known. Such networks are capable of providing communication services over wide areas and may include, in addition to basic telephone service, such services as call forwarding, call transfers, high speed data transfers, etc.
ISDN features may be accessed from a telephone console or interconnected data processor. ISDN features may be specified for each transaction or may be entered on a semipermanent basis and changed only as needed.
ISDN services may be procured over bearer ("B") channels and associated signalling ("D") channels. The B channel typically allows data transfers at a 64 kilobyte/second rate. The D channel typically transfers control information at a 16 kilobyte/second rate. The D channel is typically used for system set-up and for control of B channels.
B channels under ISDN may be offered in pairs within a basic rate interface (BRI) channel allowing a maximum data transfer rate of 144 kilobyte/second over a single data link including two B channels and an associated D channel. The BRI is also typically referred to as a 2B+D channel as a reflection of included data channels. Examples of such channel configurations and usage may be better understood by reference to the publication "ISDN '90 Proceedings" (available from Information Gatekeepers, Inc., 214 Harvard Ave., Boston, Mass. 02134) incorporated herein by reference.
Subscriber access to a BRI channel may be obtained through use of a terminal adapter designed for interconnecting a BRI to subscriber equipment accessible by an end user. The function of a terminal adapter is typically to separate (de-interleave) BRI channels into useable data streams for subscriber equipment. Subscriber equipment may include such devices as telephones, personal computers, fax machines, etc.
Shown, generally, in FIG. 1 is a prior art terminal adapter for interconnecting a BRI to end user. Such a terminal adapter may be used to support exchange of voice information on a B1 channel and data on a B2 channel (for a better understanding of the FIG. 1 terminal adapter, please see publication no. MC145488/D available from Motorola, Inc.).
Within such a terminal adapter the BRI (B1, B2, and D channels) may be interconnected with the terminal adapter through an "ISDN S/T INTERFACE". Voice information on a B1 channel may be exchanged through the terminal adapter with a telephone connected to a terminal labeled "TO TELEPHONE HEADSET". Data on a B2 channel may be exchanged with a subscriber's computer through a terminal labeled "TO EIA-232 TERMINAL". BRI control information (D channel information) may be exchanged with an operator from a keyboard/display through an interconnect on the terminal adapter labeled "KEYPAD & DISPLAY".
Terminal adapters, as is known in the art, are microprocessor intensive in construction. The terminal adapter (FIG. 1), as shown, may require the use of at least three microprocessors (MC68HC05, DDLC, and MC68000) for separation of B1 and B2 channels. More complicated ISDN interfaces where the B1 and B2 channels service differing devices (with control functions imposed on a common D channel) may require the use of a digital signal processor (DSP) such as the Motorola DSP56000, available from Motorola, Inc, for proper exchange of information.
Cellular radiotelephone systems are also known. Such cellular systems typically consist of a number of cellular base sites (each with a service coverage area) providing cellular communication services over a wide area. Cellular telephones within the service coverage area of a base site may request and be granted access to the cellular system based upon information exchanged between the cellular telephone and base site. Cellular telephones may also request access to other, target cellular telephones in the same or other service coverage area served by the same or another base site.
Target cellular telephones typically are accessed through the generation of paging requests transmitted from selected base sites within a system. Target communication units receiving such requests typically respond identifying their presence within a particular service coverage area.
A cellular telephone may identify itself to a proximate base site through transmission of an access request or through a response to a paging request. In either case the cellular base site may proceed to set up a traffic channel between the cellular communication unit and base site.
The traffic channel set up between the communication unit and base site is typically spectrally limited. Such spectral limitation places limitations on information transfer rates between communication unit and base sites over an air-interface limitations on the order of 32 kbytes/second are typical of cellular communication units.
Because of spectral limitations on information transfer between cellular communication units and base sites, information transfers may be accommodated through an ISDN interconnect with a cellular base site but limited to a single B channel per traffic channel. Also, because of spectral limitations the 64 kbyte/s exchange rate of a B channel must be transcoded to a lower exchange rate (e.g. 32 kbyte/s). The second B channel within a BRI may be allocated by the base site to serve a second communication transaction involving a second communication unit, with the base site allocated the responsibility of generating certain control commands required in setting-up and controlling the BRI.
Also, because of spectral limitations, a B channel assigned to exchanging a communicated signal with a cellular communication unit must be transcoded to a lower information exchange rate to accommodate air-interface limitations between base sites and cellular communication units. Because of the lower information exchange rate and dissimilar data structures the base site must receive and reformat information exchanged between the BRI and communication unit.
Because of the importance of ISDN services a need exists for a simpler method of separating B channels at base sites. Such method should accommodate the independent allocation of B channels yet still retain the common control structure of the common D channel.